Non-aqueous pigment dispersions using dispersion synergists

ABSTRACT

Non-aqueous pigment dispersions exhibit improved dispersion quality and/or stability for a specific selection of naphthol AS pigments and azo pigments by using specific quinacridone dispersion synergists. The non-aqueous pigment dispersions can be advantageously used in inkjet inks inkjet printing methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application ofPCT/EP2010/065758, filed Oct. 20, 2010. This application claims thebenefit of U.S. Provisional Application No. 61/259,664, filed Nov. 10,2009, which is incorporated by reference herein in its entirety. Inaddition, this application claims the benefit of European ApplicationNo. 09174945.7, filed Nov. 3, 2009, which is also incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pigmented dispersions and inkjet inkscomprising colour pigments that are stabilized by polymeric dispersantsin a non-aqueous medium using specific dispersion synergists.

2. Description of the Related Art

A dispersant is a substance for promoting the formation andstabilization of a dispersion of pigment particles in a dispersionmedium and for substantially reducing the dispersing energy required.Dispersed pigment particles may have a tendency to re-agglomerate afterthe dispersing operation, due to mutual attraction forces. The use ofdispersants counteracts this re-agglomeration tendency of the pigmentparticles.

The dispersant has to meet particularly high requirements when used forinkjet inks. Inadequate dispersing manifests itself as increasedviscosity in liquid systems, loss of brilliance and/or hue shifts.Moreover, particularly good dispersion quality is required to ensureunimpeded passage of pigment particles through the nozzles of a printhead, which are usually only a few micrometers in diameter. In addition,pigment particle agglomeration and the associated blockage of theprinter nozzles has to be avoided in the standby periods of the printer.

Polymeric dispersants typically contain in one part of the moleculeso-called anchor groups, which adsorb onto the pigments to be dispersed.In a spatially separate part of the molecule, polymeric dispersants havepolymer chains compatible with the dispersion medium, thus stabilizingthe pigment particles in the dispersion medium. Typical polymericdispersants include graft copolymer and block copolymer dispersants.

In aqueous inkjet inks, the polymeric dispersants generally containhydrophobic anchor groups exhibiting a high affinity for the pigmentsurface and hydrophilic polymer chains for stabilizing the pigments inthe aqueous dispersion medium.

The preparation of good thermally stable dispersions with submicronparticles is more difficult for non-aqueous pigment dispersions, such assolvent based, oil based and radiation curable inkjet inks. The pigmentsare especially difficult to disperse when they have a non-polar surface.

These problems have lead to the design of very specific polymericdispersants wherein the anchor groups are pigment derivatives. Forexample, WO 2007/006635 (AGFA GRAPHICS) discloses pigment dispersionscomprising a colour pigment and a polymeric dispersant having via alinking group covalently linked to its polymeric backbone at least onepending chromophore group which has a molecular weight smaller than 90%of the molecular weight of the colour pigment. A disadvantage is thatthe pending chromophore group must bear some similarity to the chemicalformula of the pigment in order to obtain effective dispersion, which asa consequence results in different polymeric dispersants for thedifferent pigments used in an ink set containing yellow, magenta andcyan inks.

Another approach for dispersing pigments with non-polar surfaces innon-aqueous dispersion media is changing the surface to a more polarsurface by addition of compounds known as dispersion synergists. Adispersion synergist is a compound that promotes the adsorption of thepolymeric dispersant on the surface of the pigment. To be efficient, thesynergist should bear some similarity to the pigment. Examples of thesedispersion synergists are given in, for example, WO 2007/060254 (AGFAGRAPHICS), EP 1790697 A (AGFA GRAPHICS) and EP 1790698 A (AGFAGRAPHICS).

However, both approaches lead to a considerable higher cost in composingnon-aqueous pigment dispersions for inkjet ink sets, because eachpigment dispersion requires anchor groups on the polymeric dispersant ordispersion synergists which chemically resemble the pigments.

For consistent image quality, inkjet inks require a dispersion stabilitycapable of dealing with high temperatures (above 60° C.) duringtransport of the ink to a customer, jetting at elevated temperatures andchanges in the dispersion medium of the inkjet ink during use, forexample, evaporation of solvent and increasing concentrations ofhumectants, penetrants and other additives.

Therefore, it is highly desirable to manufacture low cost non-aqueouspigment dispersions, especially pigmented inkjet inks, exhibiting a highdispersion quality and stability using the same polymeric dispersant inall inks of an ink set.

SUMMARY OF INVENTION

In order to overcome the problems described above, it has beensurprisingly found that non-aqueous pigment dispersions, especiallynon-aqueous inkjet inks, of high dispersion quality and stability wereobtained by a combination of a specific quinacridone dispersionsynergist and a specific naphthol AS pigment or azo pigment as definedbelow. The magenta colour of the dispersion synergist had no negativeeffect on the colour of yellow pigment dispersions. CMYK inkjet ink setscould be made using the same magenta quinacridone dispersion synergistand polymeric dispersant in the yellow and magenta inkjet inks.

Further preferred embodiment of the present invention produce images ofhigh image quality with non-aqueous pigment dispersions, especially withnon-aqueous inkjet inks.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Definitions

The term “C.I.” is used in disclosing the present application as anabbreviation for Colour Index.

The term “alkyl” means all variants possible for each number of carbonatoms in the alkyl group i.e. for three carbon atoms: n-propyl andisopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl;for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyland 2-methyl-butyl etc.

Non-Aqueous Pigment Dispersions and Inkjet Inks

The non-aqueous pigment dispersion according to a preferred embodimentof the present invention includes a non-aqueous dispersion medium, apolymeric dispersant, a pigment and a dispersion synergist, wherein thedispersion synergist is represented by Formula (I):

with R¹ and R² each independently selected from the group consisting ofhydrogen, alkyl and halogen; and wherein the pigment is selected fromthe group consisting of an azo pigment containing a benzimidazolonegroup, an azo pigment containing a quinoxalinedione group, C.I. PigmentRed 170, C.I. Pigment Red 210 and mixed crystals thereof.

In one preferred embodiment, the non-aqueous pigment dispersion includesa non-aqueous dispersion medium consisting of organic solvents.

The non-aqueous pigment dispersion can be advantageously used in paints,lacquers, e.g. automotive lacquers, and printing inks for applicationsin inkjet, offset, flexography and the like. The non-aqueous pigmentdispersion is preferably curable by UV radiation or e-beam.

In one preferred embodiment, the non-aqueous pigment dispersion is aninkjet ink, more preferably an inkjet ink curable by UV radiation ore-beam.

In a preferred embodiment, the non-aqueous inkjet ink is part of aninkjet ink set, preferably an inkjet ink set including two or moreinkjet inks in accordance with preferred embodiments of the invention.Preferably the inkjet ink set contains a yellow inkjet ink and a magentainkjet ink in accordance with preferred embodiments of the presentinvention.

The non-aqueous inkjet ink forms preferably part of a non-aqueous CMY(K)inkjet ink set. The non-aqueous CMY(K) inkjet ink set may also beextended with extra inks such as red, green, blue, and/or orange tofurther enlarge the colour gamut of the image. The CMY(K) ink set mayalso be extended by the combination of full density and light densityinks of both colour inks and/or black inks to improve the image qualityby lowered graininess. The preferred pigments in a cyan ink are C.I.Pigment Blue 15:3 and 15:4. The preferred pigment in a black ink iscarbon black.

The non-aqueous pigmented dispersion according to a preferred embodimentof the present invention may further also contain at least onesurfactant to control the homogenous spreading of the pigment dispersionon a substrate. For a non-aqueous pigmented inkjet ink, the surfactantis important to control the dot size of the ink droplet on a substrate.

In a preferred embodiment the non-aqueous pigmented dispersion is anon-aqueous inkjet ink containing at least one humectant to prevent theclogging of the nozzle, due to its ability to slow down the evaporationrate of ink.

The viscosity of an inkjet ink is preferably lower than 30 mPa·s, morepreferably lower than 15 mPa·s, and most preferably between 2 and 10mPa·s at a shear rate of 100 s⁻¹ and a jetting temperature between 10and 70° C.

The inkjet ink can be advantageously used in an inkjet printing methodcomprising the steps:

a) providing an inkjet ink according to a preferred embodiment of thepresent invention; and

b) jetting the inkjet ink onto an ink-receiver. In a more preferredembodiment, the inkjet printing method uses a yellow inkjet ink and amagenta inkjet ink both according to preferred embodiments of thepresent invention.

Pigments

The pigment used in a preferred embodiment of the present invention isselected from the group consisting of an azo pigment containing abenzimidazolone group, an azo pigment containing a quinoxalinedionegroup, C.I. Pigment Red 170, C.I. Pigment Red 210 and mixed crystalsthereof.

In a preferred embodiment, the pigment is selected from the groupconsisting of C.I. Pigment Yellow 120, C.I. Pigment Yellow 180, C.I.Pigment Yellow 194 and C.I. Pigment Yellow 213 and mixed crystalsthereof.

The pigments are preferably present in the range of 0.05 to 20%, morepreferably in the range of 0.1 to 10% by weight and most preferably inthe range of 1 to 6% by weight, each based on the total weight of thepigment dispersion or inkjet ink.

Dispersion Synergists

The dispersion synergists used in the non-aqueous pigment dispersionsare known from WO 2007/060254 (AGFA GRAPHICS) for dispersingquinacridone pigments having a similar chemical structure. The synthesisof these dispersion synergists is described in WO 2007/060254 (AGFAGRAPHICS), which therefore is incorporated herein as a specificreference.

The dispersion synergist used in a preferred embodiment of the presentinvention is represented by Formula (I):

with R¹ and R² each independently selected from the group consisting ofhydrogen, alkyl and halogen

In a preferred embodiment, the dispersion synergist is represented byFormula (II):

with R¹ and R² each independently selected from the group consisting ofhydrogen, alkyl and halogen.

Another preferred dispersion synergist is the quinacridone derivativeaccording to Formula (I) or (II) wherein R¹ and R² are both hydrogen.

Another preferred dispersion synergist is the quinacridone derivativeaccording to Formula (I) or (II) wherein R¹ and R² are both a methylgroup.

Another preferred dispersion synergist is the quinacridone derivativeaccording to Formula (I) or (II) wherein R¹ represents hydrogen and R²represents methyl.

Another preferred dispersion synergist is the quinacridone derivativeaccording to Formula (I) or (II) wherein R¹ and R² are both a chlorogroup.

The most preferred dispersion synergist is represented by Formula (III):

The dispersion synergist in accordance with a preferred embodiment ofthe present invention is used to improve the dispersion quality and/ordispersion stability of a pigment selected from the group consisting ofan azo pigment containing a benzimidazolone group, an azo pigmentcontaining a quinoxalinedione group, C.I. Pigment Red 170, C.I. PigmentRed 210 and mixed crystals thereof. More preferably, the dispersionsynergist in accordance with a preferred embodiment of the presentinvention is used to improve the dispersion quality and/or dispersionstability of a pigment selected from the group consisting of C.I.Pigment Yellow 120, C.I. Pigment Yellow 180, C.I. Pigment Yellow 194 andC.I. Pigment Yellow 213.

The synergist should be additional to the amount of polymericdispersant(s). The ratio of polymeric dispersant/dispersion synergistdepends upon the pigment and should be determined experimentally.Typically the ratio wt % polymeric dispersant/wt % dispersion synergistis selected between 2:1 to 100:1, preferably between 2:1 and 20:1.

Non-Aqueous Dispersion Media

The dispersion medium used in the pigment dispersion according to apreferred embodiment of the present invention is a non-aqueous liquid.The dispersion medium may consist of organic solvent(s), but may also bea radiation curable liquid which can be cured by UV-radiation or e-beam.

Suitable organic solvents include alcohols, aromatic hydrocarbons,ketones, esters, aliphatic hydrocarbons, higher fatty acids, carbitols,cellosolves, higher fatty acid esters. Suitable alcohols includemethanol, ethanol, propanol and 1-butanol, 1-pentanol, 2-butanol,t.-butanol. Suitable aromatic hydrocarbons include toluene, and xylene.Suitable ketones include methyl ethyl ketone, methyl isobutyl ketone,2,4-pentanedione and hexafluoroacetone. Also glycol, glycolethers,N-methylpyrrolidone, N,N-dimethylacetamid, N,N-dimethylformamid may beused.

Suitable examples of organic solvents are disclosed in [0133] to [0146]of EP 1857510 A (AGFA GRAPHICS).

If the pigment dispersion is a curable pigment dispersion or inkjet ink,organic solvent(s) are preferably fully replaced by one or more monomersand/or oligomers to obtain the liquid dispersion medium. Sometimes, itcan be advantageous to add a small amount of an organic solvent toimprove the dissolution of the dispersant. The content of organicsolvent should be lower than 20 wt %, more preferably lower than 5 wt %based on the total weight of the pigmented inkjet ink and mostpreferably the curable pigment dispersion doesn't include any organicsolvent.

For oil based pigment dispersions and inkjet inks, the dispersion mediumcan be any suitable oil including aromatic oils, paraffinic oils,extracted paraffinic oils, naphthenic oils, extracted napthenic oils,hydrotreated light or heavy oils, vegetable oils and derivatives andmixtures thereof. Paraffinic oils can be normal paraffin types (octaneand higher alkanes), isoparaffins (isooctane and higher iso-alkanes) andcycloparaffins (cyclooctane and higher cyclo-alkanes) and mixtures ofparaffin oils.

Suitable examples of oils are disclosed in [0151] to [0164] of EP1857510 A (AGFA GRAPHICS).

Monomers and Oligomers

The monomers and oligomers used in radiation curable pigment dispersionsand inks, especially for food packaging applications, are preferablypurified compounds having no or almost no impurities, more particularlyno toxic or carcinogenic impurities. The impurities are usuallyderivative compounds obtained during synthesis of the polymerizablecompound. Sometimes, however, some compounds may be added deliberatelyto pure polymerizable compounds in harmless amounts, for example,polymerization inhibitors or stabilizers.

Any monomer or oligomer capable of free radical polymerization may beused as polymerizable compound. A combination of monomers, oligomersand/or prepolymers may also be used. The monomers, oligomers and/orprepolymers may possess different degrees of functionality, and amixture including combinations of mono-, di-, tri- and higherfunctionality monomers, oligomers and/or prepolymers may be used. Theviscosity of the radiation curable compositions and inks can be adjustedby varying the ratio between the monomers and oligomers.

Particularly preferred monomers and oligomers are those listed in [0106]to [0115] in EP 1911814 A (AGFA GRAPHICS) incorporated herein as aspecific reference.

A preferred class of monomers and oligomers are vinyl ether acrylatessuch as those described in U.S. Pat. No. 6,310,115 (AGFA), incorporatedherein by reference. Particularly preferred compounds are2-(2-vinyloxyethoxy)ethyl (meth)acrylate, most preferably the compoundis 2-(2-vinyloxyethoxy)ethyl acrylate.

Polymeric Dispersants

Typical polymeric dispersants are copolymers of two monomers but maycontain three, four, five or even more monomers. The properties ofpolymeric dispersants depend on both the nature of the monomers andtheir distribution in the polymer. Copolymeric dispersants preferablyhave the following polymer compositions:

-   -   statistically polymerized monomers (e.g. monomers A and B        polymerized into ABBAABAB);    -   alternating polymerized monomers (e.g. monomers A and B        polymerized into ABABABAB);    -   gradient (tapered) polymerized monomers (e.g. monomers A and B        polymerized into AAABAABBABBB);    -   block copolymers (e.g. monomers A and B polymerized into        AAAAABBBBBB) wherein the block length of each of the blocks (2,        3, 4, 5 or even more) is important for the dispersion capability        of the polymeric dispersant;    -   graft copolymers (graft copolymers consist of a polymeric        backbone with polymeric side chains attached to the backbone);        and    -   mixed forms of these polymers, e.g. blocky gradient copolymers.

Suitable polymeric dispersants are listed in the section on“Dispersants”, more specifically [0064] to [0070] and [0074] to [0077],in EP 1911814 A (AGFA GRAPHICS) incorporated herein as a specificreference.

The polymeric dispersant has preferably a number average molecularweight Mn between 500 and 30,000, more preferably between 1,500 and10,000.

The polymeric dispersant has preferably a weight average molecularweight Mw smaller than 100,000, more preferably smaller than 50,000 andmost preferably smaller than 30,000.

The polymeric dispersant has preferably a polydispersity PD smaller than2, more preferably smaller than 1.75 and most preferably smaller than1.5.

Commercial examples of polymeric dispersants are the following:

-   -   DISPERBYK™ dispersants available from BYK CHEMIE GMBH;    -   SOLSPERSE™ dispersants available from NOVEON;    -   TEGO™ DISPERS™ dispersants from EVONIK;    -   EDAPLAN™ dispersants from MUNZING CHEMIE;    -   ETHACRYL™ dispersants from LYONDELL;    -   GANEX™ dispersants from ISP;    -   DISPEX™ and EFKA™ dispersants from CIBA SPECIALTY CHEMICALS INC;    -   DISPONER™ dispersants from DEUCHEM; and    -   JONCRYL™ dispersants from JOHNSON POLYMER.

Particularly preferred polymeric dispersants include SOLSPERSE™dispersants from NOVEON, EFKA™ dispersants from CIBA SPECIALTY CHEMICALSINC and DISPERBYK™ dispersants from BYK CHEMIE GMBH. Particularlypreferred dispersants are SOLSPERSE™ 32000, 35000 and 39000 dispersantsfrom NOVEON.

The polymeric dispersant is preferably used in an amount of 2 to 600 wt%, more preferably 5 to 200 wt %, most preferably 50 to 90 wt % based onthe weight of the pigment.

Photoinitiators

If the pigment dispersion or ink is radiation curable, preferably one ormore photoinitiators are present in the pigment dispersion or ink.

The photoinitiator is preferably a free radical initiator. A freeradical photoinitiator is a chemical compound that initiates apolymerization of monomers and oligomers when exposed to actinicradiation by the formation of a free radical.

Two types of free radical photoinitiators can be distinguished and usedin the pigment dispersion or ink in preferred embodiment of the presentinvention. A Norrish Type I initiator is an initiator which cleavesafter excitation, yielding the initiating radical immediately. A Norrishtype II-initiator is a photoinitiator which is activated by actinicradiation and forms free radicals by hydrogen abstraction from a secondcompound that becomes the actual initiating free radical. This secondcompound is called a polymerization synergist or co-initiator. Both typeI and type II photoinitiators can be used in a preferred embodiment ofpresent invention, alone or in combination.

Suitable photo-initiators are disclosed in CRIVELLO, J. V., et al.VOLUME III: Photoinitiators for Free Radical Cationic. 2nd edition.Edited by BRADLEY, G. London, UK: John Wiley and Sons Ltd, 1998. p.287-294.

Specific examples of photo-initiators may include, but are not limitedto, the following compounds or combinations thereof: benzophenone andsubstituted benzophenones, 1-hydroxycyclohexyl phenyl ketone,thioxanthones such as isopropylthioxanthone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-benzyl-2-dimethylamino-(4-morpholinophenyl) butan-1-one, benzildimethylketal, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphineoxide, 2,4,6-trimethylbenzoyldiphenylphosphine oxide,2-methyl-1-[4-(methylthio) phenyl]-2-morpholinopropan-1-one,2,2-dimethoxy-1,2-diphenylethan-1-one or 5,7-diiodo-3-butoxy-6-fluorone,diphenyliodonium fluoride and triphenylsulfonium hexafluophosphate.

Suitable commercial photo-initiators include IRGACURE™ 184, IRGACURE™500, IRGACURE™ 907, IRGACURE™ 369, IRGACURE™ 1700, IRGACURE™ 651,IRGACURE™ 819, IRGACURE™ 1000, IRGACURE™ 1300, IRGACURE™ 1870, DAROCUR™1173, DAROCUR™ 2959, DAROCUR™ 4265 and DAROCUR™ ITX available from CIBASPECIALTY CHEMICALS, LUCERIN™ TPO available from BASF AG, ESACURE™KT046, ESACURE™ KIP150, ESACURE™ KT37 and ESACURE™ EDB available fromLAMBERTI, H-NU™ 470 and H-NU™ 470™ available from SPECTRA GROUP Ltd.

Suitable cationic photo-initiators include compounds, which form aproticacids or Bronsted acids upon exposure to ultraviolet and/or visiblelight sufficient to initiate polymerization. The photo-initiator usedmay be a single compound, a mixture of two or more active compounds, ora combination of two or more different compounds, i.e. co-initiators.Non-limiting examples of suitable cationic photo-initiators arearyldiazonium salts, diaryliodonium salts, triarylsulfonium salts,triarylselenonium salts and the like.

However for safety reasons, in particular for food packagingapplications, the photoinitiator is preferably a so-called diffusionhindered photoinitiator. A diffusion hindered photoinitiator is aphotoinitiator which exhibits a much lower mobility in a cured layer ofthe curable liquid or ink than a monofunctional photoinitiator, such asbenzophenone. Several methods can be used to lower the mobility of thephotoinitiator. One way is to increase the molecular weight of thephotoinitiator so that the diffusion speed is reduced, e.g. difunctionalphotoinitiators or polymeric photoinitiators. Another way is to increaseits reactivity so that it is built into the polymerizing network, e.g.multifunctional photoinitiators and polymerizable photoinitiators. Thediffusion hindered photoinitiator is preferably selected from the groupconsisting of non-polymeric di- or multifunctional photoinitiators,oligomeric or polymeric photoinitiators and polymerizablephotoinitiators. Non-polymeric di- or multifunctional photoinitiatorsare considered to have a molecular weight between 300 and 900 Dalton.Non-polymerizable monofunctional photoinitiators with a molecular weightin that range are not diffusion hindered photoinitiators. Mostpreferably the diffusion hindered photoinitiator is a polymerizableinitiator.

A suitable diffusion hindered photoinitiator may contain one or morephotoinitiating functional groups derived from a Norrish typeI-photoinitiator selected from the group consisting of benzoinethers,benzil ketals, α,α-dialkoxyacetophenones, α-hydroxyalkylphenones,α-aminoalkylphenones, acylphosphine oxides, acylphosphine sulfides,α-haloketones, α-halosulfones and phenylglyoxalates.

A suitable diffusion hindered photoinitiator may contain one or morephotoinitiating functional groups derived from a Norrish typeII-initiator selected from the group consisting of benzophenones,thioxanthones, 1,2-diketones and anthraquinones.

Suitable diffusion hindered photoinitiators are also those disclosed inEP 2053101 A (AGFA GRAPHICS) in paragraphs [0074] and [0075] fordifunctional and multifunctional photoinitiators, in paragraphs [0077]to [0080] for polymeric photoinitiators and in paragraphs [0081] to[0083] for polymerizable photoinitiators.

A preferred amount of photoinitiator is 0-50 wt %, more preferably0.1-20 wt %, and most preferably 0.3-15 wt % of the total weight of thecurable pigment dispersion or ink.

In order to increase the photosensitivity further, the curable pigmentdispersion or ink may additionally contain co-initiators. Suitableexamples of co-initiators can be categorized in 4 groups:

(1) tertiary aliphatic amines such as methyldiethanolamine,dimethylethanolamine, triethanolamine, triethylamine andN-methylmorpholine;

(2) aromatic amines such as amylparadimethylaminobenzoate,2-n-butoxyethyl-4-(dimethylamino)benzoate,2-(dimethylamino)ethylbenzoate, ethyl-4-(dimethylamino)benzoate, and2-ethylhexyl-4-(dimethylamino)benzoate; and

(3) (meth)acrylated amines such as dialkylamino alkyl(meth)acrylates(e.g., diethylaminoethylacrylate) or N-morpholinoalkyl-(meth)acrylates(e.g., N-morpholinoethyl-acrylate).

The preferred co-initiators are aminobenzoates.

When one or more co-initiators are included into the curable pigmentdispersion or ink according to a preferred embodiment of the presentinvention, preferably these co-initiators are diffusion hindered forsafety reasons, in particular for food packaging applications.

A diffusion hindered co-initiator is preferably selected from the groupconsisting of non-polymeric di- or multifunctional co-initiators,oligomeric or polymeric co-initiators and polymerizable co-initiators.More preferably the diffusion hindered co-initiator is selected from thegroup consisting of polymeric co-initiators and polymerizableco-initiators. Most preferably the diffusion hindered co-initiator is apolymerizable co-initiator having at least one (meth)acrylate group,more preferably having at least one acrylate group.

Preferred diffusion hindered co-initiators are the polymerizableco-initiators disclosed in EP 2053101 A (AGFA GRAPHICS) in paragraphs[0088] and [0097].

Preferred diffusion hindered co-initiators include a polymericco-initiator having a dendritic polymeric architecture, more preferablya hyperbranched polymeric architecture. Preferred hyperbranchedpolymeric co-initiators are those disclosed in US 2006014848 (AGFA)incorporated herein as a specific reference.

The curable pigment dispersion or ink preferably comprises the diffusionhindered co-initiator in an amount of 0.1 to 50 wt %, more preferably inan amount of 0.5 to 25 wt %, most preferably in an amount of 1 to 10 wt% of the total weight of the ink.

Polymerization Inhibitors

The curable pigment dispersion may contain a polymerization inhibitor.Suitable polymerization inhibitors include phenol type antioxidants,hindered amine light stabilizers, phosphor type antioxidants,hydroquinone monomethyl ether commonly used in (meth)acrylate monomers,and hydroquinone, t-butylcatechol, pyrogallol may also be used.

Suitable commercial inhibitors are, for example, SUMILIZER™ GA-80,SUMILIZER™ GM and SUMILIZER™ GS produced by Sumitomo Chemical Co. Ltd.;GENORAD™ 16, GENORAD™ 18 and GENORAD™ 20 from Rahn AG; IRGASTAB™ UV10and IRGASTAB™ UV22, TINUVIN™ 460 and CGS20 from Ciba SpecialtyChemicals; FLOORSTAB™ UV range (UV-1, UV-2, UV-5 and UV-8) fromKromachem Ltd, ADDITOL™ S range (S100, 5110, 5120 and 5130) from CytecSurface Specialties.

Since excessive addition of these polymerization inhibitors will lowerthe ink sensitivity to curing, it is preferred that the amount capableof preventing polymerization is determined prior to blending. The amountof a polymerization inhibitor is preferably lower than 2 wt % of thetotal pigment dispersion or ink.

Binders

Non-aqueous pigment dispersions based on organic solvents or oilspreferably include a binder resin. The binder functions as a viscositycontrolling agent and also provides fixability relative to the polymericresin substrate, e.g. a polyvinyl chloride substrate, also called vinylsubstrate. The binder must be selected to have a good solubility in thesolvent(s).

Suitable examples of binder resins include acrylic resins, modifiedacrylic resins, styrene acrylic resins, acrylic copolymers, acrylateresins, aldehyde resins, rosins, rosin esters, modified rosins andmodified rosin resins, acetyl polymers, acetal resins such as polyvinylbutyral, ketone resins, phenolic resins and modified phenolic resins,maleic resins and modified maleic resins, terpene resins, polyesterresins, polyamide resins, polyurethane resins, epoxy resins, vinylresins, vinyl chloride-vinyl acetate copolymer resins, cellulose typeresins such as nitro cellulose, cellulose acetopropionate and celluloseacetate butyrate, and vinyl toluene-α-methylstylene copolymer resin.These binders may be used alone or in a mixture thereof. The binder ispreferably a film-forming thermoplastic resin.

The amount of binder resin in a pigment dispersion or ink is preferablyin the range of 0.1 to 30 wt %, more preferably 1 to 20 wt %, mostpreferably 2 to 10 wt % based on the total weight of the pigmentdispersion or ink.

Surfactants

The pigment dispersion or ink may contain at least one surfactant. Thesurfactant(s) can be anionic, cationic, non-ionic, or zwitter-ionic andare usually added in a total quantity less than 20 wt % based on thetotal weight of the inkjet ink and particularly in a total less than 10wt % based on the total weight of the pigment dispersion or ink.

Suitable surfactants include fluorinated surfactants, fatty acid salts,ester salts of a higher alcohol, alkylbenzene sulfonate salts,sulfosuccinate ester salts and phosphate ester salts of a higher alcohol(for example, sodium dodecylbenzenesulfonate and sodiumdioctylsulfosuccinate), ethylene oxide adducts of a higher alcohol,ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of apolyhydric alcohol fatty acid ester, and acetylene glycol and ethyleneoxide adducts thereof (for example, polyoxyethylene nonylphenyl ether,and SURFYNOL™ 104, 104H, 440, 465 and TG available from AIR PRODUCTS &CHEMICALS INC.).

For non-aqueous inkjet inks preferred surfactants are selected fromfluoro surfactants (such as fluorinated hydrocarbons) and siliconesurfactants. The silicones are typically siloxanes and can bealkoxylated, polyether modified, polyether modified hydroxy functional,amine modified, epoxy modified and other modifications or combinationsthereof. Preferred siloxanes are polymeric, for examplepolydimethylsiloxanes.

In a curable inkjet ink a fluorinated or silicone compound as disclosedabove may be used as a surfactant, but preferably a cross-linkablesurfactant is used. Polymerizable monomers having surface-active effectsinclude silicone modified acrylates, silicone modified methacrylates,acrylated siloxanes, polyether modified acrylic modified siloxanes,fluorinated acrylates, and fluorinated methacrylates. Polymerizablemonomers having surface-active effects can be mono-, di-, tri- or higherfunctional (meth)acrylates or mixtures thereof.

Humectants/Penetrants

Suitable humectants include triacetin, N-methyl-2-pyrrolidone, glycerol,urea, thiourea, ethylene urea, alkyl urea, alkyl thiourea, dialkyl ureaand dialkyl thiourea, diols, including ethanediols, propanediols,propanetriols, butanediols, pentanediols, and hexanediols; glycols,including propylene glycol, polypropylene glycol, ethylene glycol,polyethylene glycol, diethylene glycol, tetraethylene glycol, andmixtures and derivatives thereof. Preferred humectants are triethyleneglycol mono butylether, glycerol and 1,2-hexanediol.

The humectant is preferably added to the inkjet ink formulation in anamount of 0.1 to 40 wt % of the formulation, more preferably 0.1 to 10wt % of the formulation, and most preferably approximately 4.0 to 6.0 wt%.

Preparation of Pigment Dispersions and Inks

Pigment dispersions may be prepared by precipitating or milling thepigment in the dispersion medium in the presence of the dispersant.

Mixing apparatuses may include a pressure kneader, an open kneader, aplanetary mixer, a dissolver, and a Dalton Universal Mixer. Suitablemilling and dispersion apparatuses are a ball mill, a pearl mill, acolloid mill, a high-speed disperser, double rollers, a bead mill, apaint conditioner, and triple rollers. The dispersions may also beprepared using ultrasonic energy.

Many different types of materials may be used as milling media, such asglasses, ceramics, metals, and plastics. In a preferred embodiment, thegrinding media can comprise particles, preferably substantiallyspherical in shape, e.g. beads consisting essentially of a polymericresin or yttrium stabilized zirconium beads.

In the process of mixing, milling and dispersion, each process isperformed with cooling to prevent build up of heat, and for radiationcurable pigment dispersions as much as possible under light conditionsin which actinic radiation has been substantially excluded.

The pigment dispersion may contain more than one pigment, the pigmentdispersion or ink may be prepared using separate dispersions for eachpigment, or alternatively several pigments may be mixed and co-milled inpreparing the dispersion.

The dispersion process can be carried out in a continuous, batch orsemi-batch mode.

The preferred amounts and ratios of the ingredients of the mill grindwill vary widely depending upon the specific materials and the intendedapplications. The contents of the milling mixture comprise the millgrind and the milling media. The mill grind comprises pigment, polymericdispersant and a liquid carrier. For inkjet inks, the pigment is usuallypresent in the mill grind at 1 to 50 wt %, excluding the milling media.The weight ratio of pigment over polymeric dispersant is 20:1 to 1:2.

The milling time can vary widely and depends upon the pigment, theselected mechanical means and residence conditions, the initial anddesired final particle size, etc. In a preferred embodiment of thepresent invention pigment dispersions with an average particle size ofless than 100 nm may be prepared.

After milling is completed, the milling media is separated from themilled particulate product (in either a dry or liquid dispersion form)using conventional separation techniques, such as by filtration, sievingthrough a mesh screen, and the like. Often the sieve is built into themill, e.g. for a bead mill. The milled pigment concentrate is preferablyseparated from the milling media by filtration.

In general it is desirable to make inkjet inks in the form of aconcentrated mill grind, which is subsequently diluted to theappropriate concentration for use in the inkjet printing system. Thistechnique permits preparation of a greater quantity of pigmented inkfrom the equipment. By dilution, the inkjet ink is adjusted to thedesired viscosity, surface tension, colour, hue, saturation density, andprint area coverage for the particular application.

Spectral Separation Factor

The spectral separation factor SSF was found to be an excellent measureto characterize a pigmented inkjet ink, as it takes into accountproperties related to light-absorption (e.g. wavelength of maximumabsorbance λ_(max), shape of the absorption spectrum andabsorbance-value at λ_(max)) as well as properties related to thedispersion quality and stability.

A measurement of the absorbance at a higher wavelength gives anindication on the shape of the absorption spectrum. The dispersionquality can be evaluated based on the phenomenon of light scatteringinduced by solid particles in solutions. When measured in transmission,light scattering in pigment inks may be detected as an increasedabsorbance at higher wavelengths than the absorbance peak of the actualpigment. The dispersion stability can be evaluated by comparing the SSFbefore and after a heat treatment of e.g. a week at 80° C.

The spectral separation factor SSF of the ink is calculated by using thedata of the recorded spectrum of an ink solution or a jetted image on asubstrate and comparing the maximum absorbance to the absorbance at ahigher reference wavelength λ_(ref). The spectral separation factor iscalculated as the ratio of the maximum absorbance A_(max) over theabsorbance A_(ref) at a reference wavelength.

${SSF} = \frac{A_{\max}}{A_{ref}}$

The SSF is an excellent tool to design inkjet ink sets with large colourgamut. Often inkjet ink sets are now commercialized, wherein thedifferent inks are not sufficiently matched with each other. Forexample, the combined absorption of all inks does not give a completeabsorption over the whole visible spectrum, e.g. “gaps” exist betweenthe absorption spectra of the colorants. Another problem is that one inkmight be absorbing in the range of another ink. The resulting colourgamut of these inkjet ink sets is low or mediocre.

EXAMPLES Materials

All materials used in the following examples were readily available fromstandard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS(Belgium) unless otherwise specified.

PR170 is NOVOPERM™ Red F6RK which is a C.I. Pigment Red 170 pigmentavailable from CLARIANT.

PR184 is PERMANENT™ Rubine F6B which is a C.I. Pigment Red 184 pigmentavailable from CLARIANT.

PR210 is PERMANENT™ Red P FK which is a C.I. Pigment Red 170 pigmentavailable from CLARIANT.

PY120 is the abbreviation for C.I. Pigment Yellow 120 for whichNOVOPERM™ Yellow H2G, Clariant was used.

PY155 is the abbreviation for C.I. Pigment Yellow 155 for which Ink jetYellow 4G VP 2532 New from CLARIANT was used.

PY180 is the abbreviation for C.I. Pigment Yellow 180 for which TONER™Yellow HG from CLARIANT was used.

PY194 is the abbreviation for C.I. Pigment Yellow 194 for whichNOVOPERM™ yellow F2G from CLARIANT was used.

PY213 is the abbreviation for C.I. Pigment Yellow 213 for which Ink jetH5G LP3033 from CLARIANT was used.

S39000 is SOLSPERSE™ 39000, a polyethyleneimine core grafted withpolyester-hyperdispersant from LUBRIZOL.

DEGDEE is diethylene glycol diethyl ether.

DS is the dispersion synergist according to Formula (III) and wassynthesized in the same manner as described in WO 2007/060254 (AGFAGRAPHICS) for the synergist QAD-3 (see Example 1).

Measurement Methods

1. SSF

The spectral separation factor SSF of the ink was calculated by usingthe data of the recorded spectrum of an ink solution and comparing themaximum absorbance to the absorbance at a reference wavelength. Thereference wavelength is dependent on the pigment(s) used:

-   -   If the colour ink has a maximum absorbance A_(max) between 400        and 500 nm then the absorbance A_(ref) must be determined at a        reference wavelength of 600 nm.    -   If the colour ink has a maximum absorbance A_(max) between 500        and 600 nm then the absorbance A_(ref) must be determined at a        reference wavelength of 650 nm.    -   If the colour ink has a maximum absorbance A_(max) between 600        and 700 nm then the absorbance A_(ref) must be determined at a        reference wavelength of 830 nm. For C.I. Pigment Violet 23        pigments, the absorbance A_(ref) was determined at a reference        wavelength of 730 nm.

The absorbance was determined in transmission with a Shimadzu UV-2101 PCdouble beam-spectrophotometer. The inks were diluted with ethyl acetateto have a pigment concentration of 0.002 wt % based on the total weightof the ink.

A spectrophotometric measurement of the UV-VIS-NIR absorption spectrumof the diluted ink was performed in transmission-mode with a doublebeam-spectrophotometer using the settings of Table 1. Quartz cells witha path length of 10 mm were used and ethyl acetate was chosen as ablank.

TABLE 1 Mode Absorbance Wavelength range 240-900 nm Slit width 2.0 nmScan interval 1.0 nm Scan speed Fast (1165 nm/min) Detectorphoto-multiplier (UV-VIS)

Efficient pigmented inkjet inks exhibiting a narrow absorption spectrumand a high maximum absorbance have a value for SSF of at least 30.

2. Pigment Dispersion Stability

The dispersion stability was evaluated by comparing the SSF before andafter a heat treatment of one week at 80° C. Pigmented inkjet inksexhibiting good dispersion stability have a SSF after heat treatmentstill larger than 30.

3. Average Particle Size

The particle size of pigment particles in pigmented inkjet ink wasdetermined by photon correlation spectroscopy at a wavelength of 633 nmwith a 4 mW HeNe laser on a diluted sample of the pigmented inkjet ink.The particle size analyzer used was a MALVERN™ nano-S available fromGoffin-Meyvis.

The sample was prepared by addition of one drop of ink to a cuvettecontaining 1.5 mL ethyl acetate and mixed until a homogenous sample wasobtained. The measured particle size is the average value of 3consecutive measurements consisting of 6 runs of 20 seconds. For goodink jet characteristics (jetting characteristics and print quality) theaverage particle size of the dispersed particles is below 200 nm,preferably between 70 and 150 nm. The pigmented inkjet ink is consideredto be a stable pigment dispersion if the particle size remained below200 nm after a heat treatment of 7 days at 80° C.

Example 1

This example illustrates how the dispersion quality and stability of aspecific selection of pigments can be improved by a synergist having atotally different chemical structure than the pigments.

Preparation and Evaluation of Non-Aqueous Inkjet Inks

All inkjet inks were prepared in the same manner to obtain a compositionA or B as described in Table 2, depending on whether or not a dispersionsynergist was used.

TABLE 2 wt % of component Formulation A Formulation B Pigment 5.00 4.50Dispersion synergist — 0.50 DS S39000 5.00 5.00 DEGDEE 90.00  90.00

A pigment dispersion was made by mixing the pigment, the polymericdispersant 539000, optionally the dispersion synergist, and the organicsolvent DEGDEE with a dissolver and subsequently treating this mixturewith a roller mill procedure using yttrium-stabilized zirconiumoxide-beads of 0.4 mm diameter (“high wear resistant zirconia grindingmedia” from TOSOH Co.). A polyethylene flask of 60 mL was filled to halfits volume with grinding beads and 20 mL of the mixture. The flask wasclosed with a lit and put on the roller mill for three days. The speedwas set at 150 rpm. After milling, the dispersion was separated from thebeads using a filter cloth.

The inkjet inks Ink-1 to Ink-16 were prepared according to Table 3. Theaverage particle size and the spectral separation factor SSF weredetermined to evaluate the dispersion quality and were determined againafter a heat treatment of 1 week at 80° C. for evaluating the dispersionstability. The results are also listed in Table 3.

TABLE 3 Dispersion Dispersion quality stability Ink Pigment DS SSF Size(nm) SSF Size (nm) Ink-1 PR170 No 6 1070 9 423 Ink-2 PR170 Yes 62 107 37122 Ink-3 PR184 No 8 1400 10 531 Ink-4 PR184 Yes 14 608 20 306 Ink-5PR210 No 19 470 19 306 Ink-6 PR210 Yes 109 80 99 91 Ink-7 PY120 No 37139 28 160 Ink-8 PY120 Yes 39 135 35 133 Ink-9 PY155 No 9 922 9 734Ink-10 PY155 Yes 24 434 18 553 Ink-11 PY180 No 92 108 46 199 Ink-12PY180 Yes 102 108 88 103 Ink-13 PY194 No 169 85 62 110 Ink-14 PY194 Yes180 85 128 88 Ink-15 PY213 No 56 168 32 292 Ink-16 PY213 Yes 100 99 90103

From Table 3, it should be clear that the dispersion synergist DS iscapable to improve the dispersion quality of the naphthol AS pigmentsPR170 and PR210, but not of other naphthol AS pigments like PR184.

Similarly, the dispersion synergist DS improved the dispersion qualityof the yellow azo pigments containing a benzimidazolone group or aquinoxalinedione group. The disazo pigment PY155, bearing someresemblance to the disazo pigment PY180 and the monoazo pigments PY120and PY213 but lacking a benzimidazolone group or a quinoxalinedionegroup, could not be dispersed to an acceptable level.

Printed samples of the yellow inks with or without the magentadispersion synergist DS did not show any visible difference in colour.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

The invention claimed is:
 1. A non-aqueous pigment dispersioncomprising: a non-aqueous dispersion medium, a polymeric dispersant, apigment, and a dispersion synergist; wherein the dispersion synergist isrepresented by Formula (I):

wherein R¹ and R² are each independently selected from the groupconsisting of hydrogen, alkyl, and halogen; and the pigment is selectedfrom the group consisting of an azo pigment containing a benzimidazolonegroup, an azo pigment containing a quinoxalinedione group, and mixedcrystals thereof.
 2. The non-aqueous pigment dispersion according toclaim 1, wherein the pigment is selected from the group consisting ofC.I. Pigment Yellow 120, C.I. Pigment Yellow 180, C.I. Pigment Yellow194, C.I. Pigment Yellow 213, and mixed crystals thereof.
 3. Thenon-aqueous pigment dispersion according to claim 1, wherein thedispersion synergist is represented by Formula (II):

wherein R¹ and R² are each independently selected from the groupconsisting of hydrogen, alkyl, and halogen.
 4. The non-aqueous pigmentdispersion according to claim 1, wherein R¹ and R² are eachindependently selected from the group consisting of hydrogen, a methylgroup, and a chlorine atom.
 5. The non-aqueous pigment dispersionaccording to claim 1, wherein the non-aqueous dispersion medium consistsof organic solvents.
 6. The non-aqueous pigment dispersion according toclaim 1, further comprising a component that is curable by UV-radiationor e-beams.
 7. An inkjet ink comprising: the non-aqueous pigmentdispersion according to claim
 1. 8. An inkjet ink set comprising: two ormore inkjet inks as defined by claim
 7. 9. An inkjet printing methodcomprising the steps of: providing an inkjet ink as defined by claim 7;and jetting the inkjet ink onto an ink-receiver.
 10. The inkjet printingmethod according to claim 9, wherein the inkjet ink includes at leasttwo inkjet inks, and the at least two inkjet inks include a yellowinkjet ink and a magenta inkjet ink.